Frequency-dependent effects in power system components and subnetworks can be efficiently represented via rational function-based models that characterize the component port behavior as a function of frequency. The port behavior can be defined by alternative parameter sets (e.g., admittance (Y-), impedance (Z-), or scattering (S-) parameters). The model extraction procedure approximates the port characteristics over a desired band of frequencies via a compact rational model. This paper shows a detailed procedure for interfacing such models with electromagnetic transients simulators via a Norton equivalent and convolution, for multiport Y-, Z- and S-parameter-based models. The interface of a multiport transfer function element is also shown. The procedure is applicable for models on pole-residue and state-space form. The correctness of these model implementations is demonstrated for a small electrical circuit. Application examples are shown for subnetwork modeling from computed Y-parameters and for cable modeling from measured S-parameters.